Selective cell adhesion is crucial for many of the biological processes fundamental to multicellular life. The cadherins comprise one of the largest known families of cell adhesion proteins, and their function is critical in countless events in the morphogenesis and homeostasis of solid tissues. Nonetheless, the molecular mechanisms underlying cadherin-mediated cell adhesion and its regulation are only poorly understood. The long-term objective of this project is to understand the structure and function of cadherins at an atomic level of detail. In pursuit of this goal we will use X-ray crystallography to determine the three-dimensional structures of classical cadherins in key functional states. Function-related hypotheses derived from these structures will be tested by site-directed mutagenesis and cell-based adhesion experiments. The classical cadherins, which include N-, E-, and C-cadherins, are so far the best characterized. We and others have suggested that a critical regulatory mechanism for cadherin adhesion may be provided by cis- (same cell) dimerization. However, cadherins achieve their primary adhesive function through trans (cell to cell) interactions. The molecular details of these trans adhesive interfaces between cadherin molecules thus far remain unknown. To further elucidate the molecular basis of cadherin cell adhesion, we will (1) determine the three-dimensional structure of the whole, functional, dimeric extracellular domain of C-cadherin. While structures of small non-functional fragments of cadherins have been determined previously, we will now visualize a complete, functional extracellular domain. (2) Based on this structure, which is currently in the process of being determined, we will perform cell-based site-directed mutagenesis and adhesion experiments to test structure-based functional hypotheses derived from the structure.